This invention relates to optical communication systems and more particularly to apparatus for controlling the light intensity output of an optical source in an optical communication system.
In order to compensate for variations that occur in the light intensity output of an optical source such as a light emitting diode or a laser, the prior art has disclosed the use of negative feedback arrangements. These variations in light output occur due to changes in the ambient temperature surrounding the optical source and also due to changes that occur in the optical source with aging.
In U.S. Pat. No. 3,931,512 of Jan. 6, 1976 to A. H. Kent et al entitled "Line Data and Television Transmission", some of the light coupled from a light emitting diode to an optical fiber overspills to a photodetector which in turn is connected in a negative feedback arrangement with an amplifier to compensate for changes that occur within the optical source. If the photodetector which is utilized to sample the optical signal is mounted in close proximity to the optical source and this optical source is caused to operate at a high bit rate, a significant amount of electrical signal is coupled by way of parasitic capacitance from the driving electrical signal to the photodetector circuit. In some cases this electrical signal can be several orders of magnitude larger than the signal representing the optical information. This is particularly true in the situation shown in the A. H. Kent et al patent where the optical fiber is mounted in close proximity to the optical source in order to couple as much of the optical energy as possible directly into the optical fiber. With so much parasitic coupling in the feedback circuit, the electrical signal developed by the photodetector in response to the optical signal is completely masked by the parasitic electrical signal.
The technique of using negative feedback to control the changes that occur in the optical source becomes even more complicated when the optical source is required to deliver pulses having more than one predetermined amplitude. For example, in the ternary system disclosed in U.S. Pat. No. 3,714,437 of Jan. 30, 1973 to T. S. Kinsel, "Optical Communication System with PCM Encoding with Plural Discrete Unequally Spaced Intensity Levels", the optical signal produced at the transmitter is caused to have either one of two amplitude levels depending on the binary state to be represented.